1. Field of the Invention
This invention relates to a vibration damping device which can develop a large vibration damping performance when being applied to a suspension member for automobiles and the like.
2. Related Art Statement
In general, a shock absorber of monotube type or twin tube type is widely known as a conventional device developing the vibration damping performance when being applied to the suspension member for an automobile.
Such a shock absorber is constructed by enclosing an incompressible oil and a high pressure gas in an inside of a cylindrical body structure comprising a piston and a cylinder. The damping of vibrations through this shock absorber is attained by flowing the oil through a communicating hole formed in the piston based on relative motion between piston and cylinder.
However, the high pressure gas (15-30 kg/cm.sup.2) acting to guarantee the approaching of piston rod is filled in the inside of the cylindrical body through a free piston as a partition in addition to the oil, so that it is required to considerably make the friction force of the free piston large for the filling of the high pressure gas and consequently the smooth action of the free piston under an influence of small external force can not be ensured. Therefore, such a shock absorber has a problem that high frequency slight vibration can not be absorbed effectively.
In the shock absorber, the free piston slidingly comes into contact with the cylinder under an influence of a large friction force and also the piston rod slidingly comes into contact with the end face of the cylinder under an influence of a large friction force for the complete filling of oil, so that when force transmitted to the absorber does not exceed these friction forces, the vibration damping performance can not effectively be developed.